Conventional washing machines typically include a spin basket or “tub” that holds articles (e.g., clothing) to be washed. An agitator is typically disposed within the basket, and a motor provides the drive for the basket and agitator. The motor is typically a variable speed motor (such as a variable speed AC induction motor), which is also reversible to carry out certain wash cycle functions. For example, the motor may rotate in a first direction during the agitation mode and in a second, opposite mode in the spin cycle. Other motor types have also been used in washing machines for various reasons, including permanent magnet motors such as three-phase electronically commutated (EC) motors.
The typical wash cycle of a washing machine includes various sequential operational modes, such as fill, drain and spin, agitation, and spin. Braking of the basket or agitator can occur before, during or after the various modes, and the braking characteristics may be dictated by the wash cycle parameters and/or safety standards, such as Underwriters Laboratory (UL) standards. In addition, there are various instances wherein braking of the basket from a normal operation speed to a reduced non-zero speed may be desired. For example, a load imbalance during the spin cycle may require a reduced speed to prevent damage to the machine. Certain “safe” modes of the washing machine resulting from faults or other detected abnormal conditions may require braking the basket to a reduced speed. As the basket coasts to a stop after the spin mode, the basket may pass through one or more resonant/harmonic frequencies, generating excessive noise and vibration. It may be desired to apply a temporary braking torque to the motor so that the basket passes quickly through the resonant frequencies.
Various braking methods and associated hardware are known for washing machines, including mechanical braking systems and electrically induced braking torque methods. The mechanical systems that use brake pads or shoes to bring a fully loaded rotating basket to zero speed are costly to implement and maintain. The brake shoes/pads have a limited design life and will eventually wear and need replacement. The wear rate will depend on a number of factors (i.e., load size, water level in tub, frequency of use, etc.) and will vary from one machine to another.
“Dynamic braking” refers to various methods for controlling power to the motor such that the stator field rotates at a frequency that is less than the rotational frequency of the rotor, thus generating a braking torque on the rotor. These methods turn the motor into a generator and the regenerated power is dissipated via a braking resistor. This method is deemed “dynamic” in that the braking torque is proportional to the kinetic energy in the motor load. However, as the load diminishes, the braking torque also decreases. Thus, dynamic braking systems often include a different “finishing” brake to bring the motor to a complete stop, such as a mechanical brake.
“Regenerative braking” is essentially the same concept as dynamic braking except, rather than being dissipated, the regenerated power is converted back to machine electrical power via a line synchronization technique.
The dynamic and regenerative braking methods thus require braking resistors and line synchronization circuitry/hardware, which results in an increased cost per machine. For example, the use of braking resistors impacts component sizing in the control circuit and the overall cost of such circuit.
DC injection braking is a method for braking synchronous or asynchronous motors wherein DC voltage is applied to the stator windings to produce a stationary magnetic field. The spinning rotor is magnetically drawn to this stationary magnetic field, which acts as a drag (i.e., a braking force) on the rotor and will eventually stop rotation of the motor. DC injection braking has certain benefits in that it is relatively inexpensive to implement, particularly in variable frequency drives (VFD) wherein DC power is already inherently generated. However, DC injection braking has not been used in washing machines over the full operational loads and speeds of the machines due to the relatively large induced current spikes (and resulting thermal stresses) generated in the motor at higher loads and speeds. The decreased motor life resulting from the stress of repeated DC injection braking over the typical life cycle of a washing machine has virtually eliminated DC injection braking as the sole braking method for conventional washing machines.
The published U.S. Patent Application No. 2008/0295543 describes a two-phase braking method for a washing machine utilizing an AC induction motor. Initially, the motor is braked in a “reverse frequency” mode (sometimes referred to as “plugging”) to slow the motor to a first slow speed. In this mode, the stator electrical field is switched to rotate in the opposite direction of the rotating rotor and little regenerative power is produced. Once the motor has slowed, it is then braked to a stop in a DC braking mode.
U.S. Pat. No. 4,305,030 describes a braking method for an AC induction motor wherein a DC braking current is quickly supplied to the motor when AC power is disconnected to cause an immediate and rapid decrease in motor speed, as well as to prevent activation of a mechanical brake. Immediately upon disconnecting the AC power, a control circuit causes a capacitor to discharge and effectuate an immediate turn-on of the DC braking current with a large initial amplitude of DC current. This rapid turnover is followed by a smaller value of DC braking current for a controlled period of time. Although this method utilizes DC braking over the full range of motor speeds, the system would not be particularly useful for the repeated starts and stops of a washing machine motor. The repeated rapid and sudden charge of initial DC braking current will cause potentially damaging current spikes and significantly shorten the life of the motor and electronics in any washing machine.
Accordingly, the industry would benefit from a braking methodology that takes advantage of the inherent benefits of DC braking of motors to reduce the speed of a washing machine motor from a first operational speed to a lower operational speed.